Door and Window Casings

Casing Basics
-- Before installing, inspect the side and head jambs, the boards that line the edges in the wall opening and run perpendicular to the casing. The jamb edges should be flush with the wall surfaces.
--Leave a narrow portion (less than 1/4 inch) of the jamb edges exposed, this is the reveal.
-- Fit the lower trim components first. For doors, this means the plinth blocks at the base. For windows, it means installing the stool (horizontal shelf) and apron (vertical piece beneath window) before fitting the side jambs.

Casing Aids
This plinth block (an excellent aid to novice carpenters) at the base of door casing is where the side casing and baseboard meet. The moldings are cut square to butt against the plinth.
If the Shoe Fits
The gap where baseboard and floor meet is covered by a small piece of quarter-round molding known as a shoe.
Corner Adjustments
Test-fit outside corner pieces to reveal a gap that shows how much you'll need to adjust one of the angle cuts to compensate for the not-quite-square corner. If the joint is open on the outside as shown, cut both pieces at slightly less than 45 degrees. If the joints open at the back, cut both pieces at slightly more than 45 degrees.

Scarf Joint
For long wall sections that require multiple lengths of stock, splice the molding using a scarf joint, a simple overlapping of 45-degree bevel ends, positioned at a stud. Here, the left side of the joint is beveled away from the walls. The right side is beveled toward the wall. Attach the left piece and test fit the right piece. Apply wood glue and press the joint together. Drill two pilot holes through both ends (slightly angled into the joint), start the nails, and drive them in.